To view PDF documents from Firefox on a Macintosh, you will need to set your browser preferences to open PDF files using Adobe Acrobat Reader.

In Firefox, under the Firefox menu, select Preferences.
In the Preferences window, select Applications.
Under Content Type, search or browse for PDF file click on it.
Then on the right under Action, select Use Adobe Reader.
If that is not an option, select Use other, and browse to the Adobe Reader application under Applications and select Open.
Close the Preferences window and you should now be able to open PDF documents.

Another solution is to download the PDF file and change the file extension from .ashx to .pdf. Then you can view the fiew using Preview or Adobe Reader.